202 Systematic Paleontology 



A large sauropod rib (No. 5010 d, G. C.) may be referred to this 

 species. It is wide at the proximal end, with a prominent tuberculum, 

 and in section is L-shaped at first, broadening out in a shallow U-shape 

 toward the distal end. 



Length, estimated 1300 mm. 



Distance across articular ends 335 mm. 



Occurrence. — Aeundel Foemation. Near Muirkirk, Prince George's 

 County. 



Collections. — U. S. National Museum, Goucher College. 



Genus ASTRODON Johnston 



AsTEODON JOHNSTONi Lcidy 



Plate XIX, Fig. 5 



Astrodon johnstoni Leidy, 1865, Smithsonian Contrib. to Knowledge, vol. 

 xiv, art. vi, pp. 102, 119, pi. xiii, figs. 20-23, pi. xx, fig. 10. 



Description. — The type (No. 798, Yale Museum) consists of a nearly 

 perfect tooth and the section of another prepared for the microscope. 

 It is thus described by Leidy : " The tooth of Astrodon, .... bears con- 

 siderable resemblance in form to the teeth referred to the Hylceosaurus, 

 an associate of the Iguanodon and Megalosaurus, in the Wealden forma- 

 tion of Europe. The specimen comprises nearly the length of the crown, 

 and is about an inch and a half long. The shaft of the crown is straight, 

 compressed cylindroid, in transverse section ovate, the outer side strongly 

 convex, the inner side much less so. The summit of the crown is com- 

 pressed conical,' curved inward, convex externally, depressed internally, 

 and sub-acute at the lateral borders, one of which is worn in the specimen 

 so as to expose a narrow tract of dentine. 



" The transverse section of the tooth beneath the microscope, .... 

 exliibits an interior disk of dentine, with a multitude of minute tubuli 

 radiating from the narrow elliptical section of the pulp cavity, sur- 

 rounded by a thick layer of enamel." 



The dimensions are : 



Breadth of crown 12.3 mm. 



Thickness 10.7 mm. 



Length of crown 30.0 mm. 



